Carbon fibers, production process therefor and electrode for batteries

ABSTRACT

Fine carbon fibers having a fiber diameter of 1 μm or less, an interlayer distance d 002  between carbon layers as determined by an X-ray diffraction method of 0.335 to 0.342 nm or less and satisfying d 002 &lt;0.3448-0.0028 (log φ) where 4 stands for the diameter of the carbon fibers, and a thickness Lc of the crystal in the C axis direction of 40 nm or less. Fine carbon fibers containing boron in crystals of the fiber. The fine carbon fibers can be produced by using vapor-grown fine carbon fibers as a raw material, adding a boron or a boron compound to the material; pressing the mixture to regulate the bulk density to preferably 0.05 g/cm 3  or more; and heating at a temperature of 2000° C. or higher.

REFERENCE TO THE RELATED APPLICATION

[0001] This application is entitled to the benefit of the priority ofU.S. Provisional Patent Application Ser. No. 60/145,266 filed on Jul.26, 1999.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates to fine carbon fibers (includingcoil-like carbon fibers, vapor grown carbon fibers, whisker carbonfibers, expanded carbon fibers and other fibrous carbons) and a processfor producing the same, which carbon fibers are used as a filler addedto any of a variety of materials such as metals, resins, and ceramics soas to improve electric and thermal conductivity thereof, as anelectron-emitting element for an FED (field-emission display), and as afiller such as a property-improving material for a variety of batteries.The invention also relates to an electrode for batteries; i.e., apositive and a negative electrode for any of a variety of batteries suchas a dry battery, a Pb secondary battery, a capacitor, and an Li ionsecondary battery, the electrodes being incorporated with fine carbonfibers so as to improve the charge-discharge capacity and the mechanicalstrength of an electrode plate.

BACKGROUND OF THE INVENTION

[0003] Fine carbon fibers, as referred to in the present invention, aretypically produced through a vapor phase method comprising thermaldecomposition of hydrocarbon (e.g., disclosed in Japanese PatentApplication Laid-Open (kokai) Nos. 7-150419, 5-321039, 60-215816, and61-70014 and Japanese Examined Patent Publication (kokoku) Nos. 5-36521and 3-61768). The thus-produced carbon fibers typically have a diameterof approximately 0.01-5 μm. If the diameter is 0.01 μm or more, the finecarbon fibers include carbon nano-tubes and carbon nano-fibers having aconcentric or tree-annual-ring structure similar to the carbon fibersgrown by the vapor phase method.

[0004] There has been proposed use of fine carbon fibers as a filler formetals, resins, ceramics, etc. Of these, fine carbon fibers areparticularly proposed as a filler for batteries, because the developmentof portable apparatuses such as small-sized cellular phones, videocameras, and notebook-type personal computers has been remarkable, andthe demand for a small-sized secondary battery including an Li ionsecondary battery (Li battery) serving as a power source therefor hasincreased drastically.

[0005] Examples of typical carbonaceous materials for a negativeelectrode of an Li battery include hard carbon pitch, meso-phase carbonmicrobeads (MCMB), meso-phase pitch carbon fiber (MPCF), artificialgraphite, coke, and naturally occurring graphite. Furthermore, theaddition of carbon fibers produced from pitch or vapor-grown carbonfibers to these negative electrode materials has also been proposed,while carbonaceous materials such as graphite micropowder and carbonblack are incorporated into a positive electrode as electricconductivity-providing agents.

[0006] In a negative electrode of an Li battery, intercalation anddeintercalation of lithium ions occur during charge and dischargeprocesses. Graphite, having a layered structure, easily undergoesintercalation wherein a reactant (e.g., Li) is inserted into aninterlayer void to thereby expand the void (intercalation). The reactionproduct containing a compound inserted in the interlayer void is calleda graphite intercalation compound. The intercalation compound easilyreleases the intercalated reactant (deintercalation), to therebytransform itself into graphite; i.e., the original material. Fine carbonfibers, which is a material having excellent electric and thermalconductivity and an intercalating property, do not lower the capacity ofthe battery, and thus addition thereof is of interest as an additive ina negative electrode material.

[0007] The enhancing of the intercalating property of the negativeelectrode is essential for enhancing the capacity of an Li battery. Ingeneral, in order to enhance the intercalating property, the degree ofgraphitization; i.e., crystallinity, of carbonaceous materials includingfine carbon fibers must be enhanced.

[0008] A negative electrode of a lead secondary battery per se comprisesa compound having poor electric conductivity, and carbonaceous materialssuch as carbon black, graphite microparticles and carbon fiber mayoptionally be added thereto to enhance the conductivity of the negativeelectrode. Such carbonaceous materials desirably have high electricconductivity and crystallinity. In order to enhance the crystallinity ofsuch carbonaceous materials, the materials are typically treated at hightemperature in order to effect graphitization.

[0009] Meanwhile, fine carbon fibers having a small average fiberdiameter, particularly 1 μm or less, have a low bulk density and providean insufficient filling density. Thus, when fine carbon fibers areadded, to an electrode, in a large amount, the density of the electrodedecreases. Therefore, fine carbon fibers are typically added in anamount of 20 mass % or less, preferably 10 mass % or less. From thispoint of view, fine carbon fibers are considered not-to impart aremarkable effect commensurate with addition thereof even when thecrystallinity of the fibers is enhanced. Therefore, no investigationother than treatment at high temperature has been carried out so as toenhance the crystallinity of fine carbon fibers. Thus, conventionallyused fine carbon fibers have insufficient crystallinity, and thedistance between two crystal layers represented by d₀₀₂ is larger than0.3385 nm.

[0010] To meet with demand for increased capacity, electrode materialswith a lower electric resistance are sought to give a large electriccurrent charge-discharge capacity.

[0011] Addition of various conductivity-imparting materials are studiedto lower the resistance of electrodes, in which it is known that afiller of a fibrous material particularly of vapor grown carbon fibersis effective. The reasons thereof include:

[0012] 1) fine fibrous materials have an aspect ratio of 100 or moreallowing a long conducting path;

[0013] 2) vapor-grown carbon fibers have a high crystallinity to give ahigh conductivity; and

[0014] 3) vapor-grown carbon fibers have a charge-discharge capacity tothereby prevent reducing the capacity of a Li battery when added.

[0015] However, commercially available fine fibrous materials having adiameter of 1 μm or less have an upper limit of the conductivity of 0.01Ω·cm as the powder resistance when measured at a density of 0.8 g/cm³and there are no available fine fibrous materials having a conductivityhigher than this.

[0016] Recently, the crystallinity of a negative electrode material hasbeen enhanced to thereby improve the charge-discharge capacity of abattery. This trend forces an additive other than a negative electrodematerial to have a high discharge capacity. Therefore, the crystallinityof carbonaceous material serving as an additive for a negative electrodemust be enhanced.

[0017] Considering the above requisite to enhance the crystallinity offine carbon fibers, the inventors have been investigated heat treatmentup to 3200° C. to enhance the crystallinity.

[0018] However, even when fine carbon fibers (vapor-grown carbon fibers)having diameters of the order of about 0.15 μm are heated to 3000° C. orhigher, the lattice constant of the interlayer distance d₀₀₂ cannot bereduced to less than 0.3385 nm.

[0019] Simultaneously, the conductivity had an upper limit of 0.01 Ω·cmas the powder resistance when measured at a density of 0.8 g/cm³.Accordingly, fine carbon fibers having a higher crystallinity and alower resistance are sought.

[0020] The supposed reason for the above failure is that vapor-growncarbon fibers have a considerably small fiber diameter and a uniquestructure wherein the core of the concentrically grown crystalline fibercomprises a hollow or amorphous portion. In addition, when the fiberdiameter is as small as 1 μm or less, maintenance of a roll-shapestructure of a carbon hexagonal network plane becomes more difficultwith decreasing distance between the structure and the center of thefiber, to thereby make crystallization difficult. Therefore, the valueof d₀₀₂ depends on the diameter of the fiber. For example, thelimitations on the interlayer distance d₀₀₂ were 0.3385 nm for the fiberwith a diameter of about 0.15 μm, 0.3400 nm for the fiber with adiameter of about 0.05 μm, 0.3415 nm for the fiber with a diameter ofabout 0.02 μm, and 0.3420 nm for the fiber with a diameter of about 0.01μm or less. Therefore, the limitation of the interlayer space d₀₀₂ is0.3385 nm and, even when the carbon fibers are heated to 3000° C. orhigher, d₀₀₂ cannot be reduced to less than 0.3385 nm.

[0021] Accordingly, in order to enhance crystallinity and to reduce d₀₀₂to 0.3385 nm or less, there must be developed a method for enhancingcrystallinity other than heat treatment.

[0022] The object of the present invention is to develop fine carbonfibers having a high crystallinity that has not been produced through aconventional method or having a high crystallinity, and to provide abattery electrode having a higher performance by containing thedeveloped carbon fibers as a filler.

SUMMARY OF THE INVENTION

[0023] Firstly, the present inventors have investigated a graphitizationcatalyst (Called also a graphitization aid or additive and hereinaftersimply called a “graphitization catalyst” or “catalyst”) so as to attainthe above-described objects.

[0024] Before now, no investigations have been conducted on controllingthe characteristics of fine carbon fibers having a fiber diameter of 1μm or less by use of a graphitization catalyst. Further, the degree ofenhancement in the crystallinity and characteristics of fine carbonfibers having such a particular crystal structure by use of agraphitization catalyst has not been elucidated.

[0025] Secondly, the present inventors have investigated treatment offine carbon fibers by use of a catalyst.

[0026] The present invention has been accomplished based on the abovestudy. The present invention provides the following.

[0027] (1) Fine carbon fibers having a diameter of 1 μm or less; aninterlayer distance d₀₀₂ between carbon layers as determined by an X-raydiffraction method in a range of 0.335 nm to 0.342 nm, further 0.3354 nmto 0.3420 nm, and satisfying d₀₀₂<0.3448-0.0028 (log φ), preferablyd₀₀₂<0.3444-0.0028 (log φ), more preferably d₀₀₂<0.3441-0.0028 (log φ),wherein 4 stands for the diameter of the carbon fibers and the units ofd₀₀₂ and φ are nm; and a thickness Lc of the crystal in the C axisdirection of 40 nm or less.

[0028] (2) Fine carbon fibers according to (1), characterized in thatthe R value of Raman spectrum is 0.5 or more and the peak width of halfheight of a peak of the spectrum at 1580 cm⁻¹ is 20-40 cm⁻¹.

[0029] (3) Fine carbon fibers having a diameter of 1 μm or less andcontaining boron in the crystals of the fibers.

[0030] (4) Fine carbon fibers according to (1) or (2), which containboron in the crystals of the carbon fibers.

[0031] (5) Fine carbon fibers according to (3) or (4), which containboron in an amount of 0.1-3 mass %.

[0032] (6) Fine carbon fibers according to any one of (1) to (5), whichhave a fiber diameter of 0.01-1 μm and an aspect ratio of 10 or more.

[0033] (7) Fine carbon fibers according to (6), wherein the resistanceas the powder when pressed to a density of 0.8 g/cm³ is 0.01 Ω·cm orless in the direction vertical to the pressing direction.

[0034] (8) Fine carbon fibers according to any one of (1) to (7), whichare vapor-grown carbon fibers.

[0035] (9) A process for producing fine carbon fibers which comprisesadding fine carbon fibers having a diameter of 1 μm or less with boronor a boron compound and heating the fine carbon fibers to 2000° C. orhigher.

[0036] (10) A process for producing fine carbon fibers which comprisesadding fine carbon fibers having a diameter of 1 μm or less with boronor a boron compound;

[0037] regulating bulk density of the fine carbon fiber to 0.05 g/cm³ ormore; and

[0038] heating the fine carbon fiber to 2000° C. or higher while saiddensity is maintained.

[0039] (11) A process for producing fine carbon fibers according to (9)or (10) wherein the amount of the added boron or boron compound is 0.1to 10 mass %.

[0040] (12) A process for producing fine carbon fibers according to anyone of (9) or (11) wherein the fine carbon fibers to which the boron orboron compound is added have a diameter of 0.01 to 1 μm and an aspectratio of 10 or more.

[0041] (13) A process for producing fine carbon fibers according to anyone of (9) or (12) wherein the fine carbon fibers to which the boron orboron compound is added are vapor-grown carbon fibers.

[0042] (14) A process for producing fine carbon fibers according to (13)wherein the fine carbon fibers, prior to said heat treatment, to whichthe boron or boron compound is to be added are fired products ofvapor-grown carbon fibers fired after growth.

[0043] (15) A process for producing fine carbon fibers according to (13)wherein the fine carbon fibers, prior to said heat treatment, to whichthe boron or boron compound is to be added are unfired products ofvapor-grown carbon fibers not fired after the growth.

[0044] (16) A battery electrode containing fine carbon fibers as recitedin any one of (1) to (8).

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0045]FIG. 1 is a graph showing the relationship between the fiberdiameter of fine carbon fibers and the interlayer distance of thegraphite crystal; and

[0046]FIG. 2 is a cross section of an apparatus for measuring theresistance as the powder of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0047] The fine carbon fibers of the present invention have excellentcrystallinity, and the interlayer distance d₀₀₂ between carbon crystallayers as obtained through an X-ray diffraction method is 0.335-0.342 nmand the thickness Lc in the C axis direction of crystal is 40 nm orless, preferably 32 nm or less.

[0048]FIG. 1 is a graph showing the function of the interlayer distanced₀₀₂ as in relation to the fiber diameter for fine carbon fibers whichhave not been treated with boron and which have been treated with boron,which measurement was conducted by the inventors. The fine carbon fiberswhich have not been treated with boron have an interlayer distance d₀₀₂greater than those represented by d₀₀₂ =0.3448-0.0028 (log φ) where φstands for the diameter of the carbon fiber, but the fine carbon fiberswhich have been treated with boron have the interlayer distance d₀₀₂substantially smaller than those represented by the same formula.Therefore, the fine carbon fibers can be defined as having an interlayerdistance d₀₀₂ as represented by d₀₀₂ <0.3448-0.0028 (log φ), preferablyd₀₀₂ <0.3444-0.0028 (log φ), more preferably d₀₀₂ <0.3441-0.0028 (logφ), where φ stands to the diameter of the carbon fibers.

[0049] In accordance with the study by the present inventors, theinterlayer distance d₀₀₂ of carbon crystal could not be made smallerthan d₀₀₂ =0.3448-0.0028 (log φ). Here, even if such small d₀₀₂ can beattained without boron treatment, such would be able to be attained onlyby conducting a special treatment or by attaining severe processconditions or the control thereof, the significance of the presentinvention, where fine carbon fibers having a small interlayer distanced₀₀₂ in the range of the present invention can be easily obtained bytreating with boron, is not lost.

[0050] The fine carbon fibers of the present invention can be carbonfibers containing boron. The boron in the carbon fibers is present inthe crystal of carbon (graphite), between the crystal layers, or in thecrystal grain boundaries, or as impurity.

[0051] The fine carbon fibers of the present invention containing boronin crystals of the fibers are novel and therefore are not limited by theranges of d₀₀₂ and Lc as above mentioned. The fibers preferably containboron, and the d₀₀₂ and the Lc preferably fall within theabove-described ranges.

[0052] Fine carbon fibers containing boron, and in which d₀₀₂ and Lcfall within the above-described ranges, can possess an R value of 0.5 ormore, as obtained from a Raman spectrum (R=ID/IG, ratio of theabsorption intensity ID at 1360 cm⁻¹ to the absorption intensity IG at1580 cm⁻¹) and a narrow peak width at half height of peak at 1580 cm⁻¹of the spectrum of 20 to 40 cm⁻¹.

[0053] The fine carbon fibers preferably have a fiber diameter of 0.01-1μm, and, in view of the development of a function as fibers, an aspectratio of 10 or more, more preferably 50 or more.

[0054] A fiber diameter of less than 0.01 μm induces poor mechanicalstrength when such fibers are used as a battery electrode or a fillerfor resin, a function as a fiber is easily lost; e.g., the fiber iseasily cut. Meanwhile, for a given amount of addition as a filler (mass%), an increase in fiber diameter has an effect of reducing the numberof fibers to be added. In this case, characteristics of fibers as afiller are insufficiently exhibited. When the fiber diameter is great,such carbon fibers do not easily enter into interparticle spaces ofgraphite particles contained as an electrode material in a batterynegative electrode comprising carbon. When carbon fibers have a fiberdiameter in excess of 1 μm, the productivity of the fibers per sedecreases considerably, to thereby disadvantageously induce an increasein production cost. Thus, the fiber diameter is preferably 1 μm or less,more preferably 0.5 μm or less.

[0055] No particular limitation is imposed on the length of the carbonfibers, and the lower limit thereof is preferably determined from thelower limit of the aspect ratio (fiber length/fiber diameter). When thefiber length is excessively long, dispersibility when the fibers areused as a filler disadvantageously decreases due to entanglement offibers. Thus, the upper limit of the fiber length is preferably 400 μm,more preferably 100 μm. For example, when the aspect ratio is 50 ormore, a fiber diameter of 0.01 μm requires a fiber length of 0.5 μm ormore, whereas a fiber diameter of 0.1 μm requires a fiber length of 5 μmor more. Also, the upper limit of the fiber length is preferably 400 μm,more preferably 100 μm.

[0056] The fine carbon fibers of the present invention having highcrystallinity can be produced by heat treating fine carbon fibers in thepresence of a boron compound. Although it is not intended to be bound bythe theory, it is considered that, by such heat treatment, boron isincorporated into carbon fibers and the catalytic action of theincorporated boron gives the fine carbon fibers of the present inventionhigh crystallinity

[0057] The amount of boron in the fine carbon fibers effective ininducing a high degree of crystallization is generally in a range of0.1-3 mass %, preferably 0.2-3 mass %. Boron is diffused through, forexample, heating at high temperature, after completion ofcrystallization to a high degree. However, the boron content may becomelower than the amount of boron added, so long as the fiber containsboron during crystallization.

[0058] The process for producing fine carbon fibers according to thepresent invention will next be described.

[0059] Carbon Fibers Serving as a Starting Material:

[0060] In the process according to the present invention, vapor-grownfine carbon fibers obtained through thermal decomposition of an organiccompound such as benzene may be used as a starting material. Forexample, the starting material can be produced through methods disclosedin Japanese Patent Application Laid-Open (kokai) Nos. 7-150419,5-321039, 60-215816, and 61-70014 and Japanese Examined PatentPublication (kokoku) Nos. 5-36521 and 3-61768. As long as the diameteris 0.01 μm or more, fine carbon fibers as carbon nano-tubes or carbonnano-fibers having a tree-annual-ring structure can be also used.Therefore, carbon nano-tubes, carbon nano-fibers or the like, having amultiple structure produced by arc discharge method, laser method, etc.can be also used.

[0061] The process for producing fine carbon fibers in a vapor growingmethod will be briefly described. A transition metal or a compoundthereof, for example, a super fine powder of a metal such as iron,nickel or cobalt or super fine particles based on ferrocene is used as aseed; the seed super fine powder or super fine particles are formed on asubstrate; a starting carbon material and optionally a carrier gas suchas hydrogen are supplied thereto; the starting carbon material isdecomposed at a high temperature; and fine fibers having a fiberdiameter of 0.01 to 1 μm or more are grown with the super fine powder orsuper fine particles functioning as seeds. The method for forming a seedincludes, on a substrate (the wall of a furnace may be used as asubstrate), applying and drying a seed particle dispersion or a seedsource solution, spraying ferrocene or the like, and forming fineparticles of iron or a compound thereof by using ferrocene in a flowablestate, and the seed may be formed on a surface of a substrate or may beformed in a flowing bed.

[0062] Moreover, because, on the surface of fine carbon fibers as grownby a vapor growing method as above, a large amount of tars and lowboiling point components are simultaneously grown and fixed thereto andhighly active iron fine particles are also present, the carbon fibersmay be heated to treat the above substances.

[0063] However, the present inventors have found that the crystallinityof the fine carbon fibers increases insufficiently through heating. Theinventors have investigated a catalyst (aid or additive) for attaining ahigh degree of crystallization. Examples of possible catalysts includeB, Al, Be, and Si. Of these, boron (B) is particularly effective.Conventionally, there have been studied enhancement of crystallinity intypical carbonaceous materials through mixing with boron and heating(e.g., “TANSO” 1996, No. 172, p89-94; and Japanese Patent ApplicationLaid-Open (kokai) Nos. 3-245458, 5-251080, 5-266880, 7-73898, 8-31422,8-306359, 9-63584, and 9-63585).

[0064] However, no investigation has been conducted for improving acharacteristic of vapor-grown fine carbon fibers having a fiber diameterof 1 μm or less through incorporation of boron. The reason is that sincesuch fine carbon fibers have a unique structure the catalytic effectprovided by boron in typical carbonaceous material has been consideredunobtainable.

[0065] That is, vapor-grown carbon fibers have a cylindrical structurehaving a plurality of concentric layers, and a crystal structure isgrown concentrically in a cross-section. The length of the fiber variesin accordance with production conditions. Since fibers having a smallfiber diameter such as approximately 0.01-1 μm exists in the form of asingle filaments or branched filaments, definite determination of thelength is difficult. Through measurement of linear portions under ascanning electron microscope, most of the fiber filaments are found tohave a fiber diameter of 5 μm or more. Since the fibers exist in theform of single long filaments or branched filaments, both long filamentsand short filaments having a length of approximately 5 μm easily form aflock having a dimension of 10 μm or more, occasionally 100 μm or more.Thus, the fiber mass has a bulk density as small as 0.05 g/cm³ or less,typically 0.01 g/cm³ or less. In addition, a flock-like structure isformed.

[0066] Vapor grown carbon fibers having a unique structure differentfrom usual carbon fibers are difficult to contact with a graphitizationcatalyst, to thereby possibly fail to attain a homogeneous introductionof boron.

[0067] In fine carbon fibers, as the diameter of the fibers becomesfiner, particularly near the center, a carbon crystal layer tends to bebent, where it was thought unlikely that the fine carbon fibers aremaintained when the crystallinity is enhanced by making the interlayerdistance of the carbon crystal smaller. In other words, it was thoughtunlikely that the crystallinity of fine carbon fibers can be enhanced byboron.

[0068] However, the inventors, as a result of intensive efforts, haveattained carbon fibers with a high cryatallinity by using boron as acatalyst (aid) in vapor grown fine carbon fibers.

[0069] In accordance with the present invention, in order to carry outdoping with boron, there is used starting carbon fibers heat treated atlow temperature, preferably at 1500° C. or lower, or more preferably asgrown and not heat treated, which are not completely crystallized andare easily doped with boron. Even when starting fibers are not heattreated, the fibers are finally heated to the graphitization temperatureduring treatment with a catalyst comprising boron (boron-introducingtreatment). Thus, starting fibers having a low crystallization degreecan be used satisfactorily. Although carbon fibers graphitized at 2000°C. or higher, preferably 2300° C. or higher, may also be used, suchgraphitization is not necessary. Simultaneous graphitization andcrystallization of carbon fibers not heat treated by use of a catalystis preferred in view of energy conservation.

[0070] Fine carbon fibers serving as a starting material may bedisintegrated or crushed in advance, to thereby facilitate handling in asubsequent step. The disintegration or crushing is sufficient if it iscarried out to a degree such that mixing with boron or a boron compoundis possible. In other words, before doping with boron the startingfibers are not necessarily processed so as to have a length suitable fora filler, since, after doping with boron, the starting fibers arefinally subjected to treatment for preparing a filler, such asdisintegration, crushing, or classification. Vapor-grown carbon fibershaving a fiber diameter of approximately 0.01-1 μm and a length ofapproximately 0.5-400 μm may be used as such. The fibers may beflocculated. The starting fibers may also be subjected to heattreatment. In this case, the treatment is preferably carried out at1500° C. or lower.

[0071] Boron or a Boron Compound:

[0072] Boron or a boron compound which is used in boron-introducingtreatment suitably has the following properties, although it is notparticularly limited. Since the treatment is carried out at 2000° C. orhigher, there is used a substance which is not vaporized through thermaldecomposition before the temperature is raised to at least 2000° C.Examples of the substance include elemental boron, B₂O₃, H₃BO₄, B₄C, BNand other boron compounds.

[0073] Typically, carbon can be doped with boron in an amount of 3 mass% or less. Thus, boron or a boron compound in a starting mixture may bein an amount of 10 mass % or less as reduced to boron atoms based oncarbon atoms, in consideration of the doping ratio. When the amount isexcessive, the cost for the treatment increases, and the additive iseasily sintered or solidified, or covers the surface of the fibers,whereby characteristics of a filler may fall, e.g., electric resistanceincreases.

[0074] The fine carbon fibers (fiber diameter of 1 μm or less), having a3-dimensional structure, are easily flocculated and have considerablylow bulk density and high porosity. In addition, since boron is added inan amount as low as 10 mass % or less, preferably 5 mass % or less,attaining homogeneous contact between the fiber and boron species isdifficult through simple mixing of the two components.

[0075] In order to effectively introduce boron, fibers and boron or aboron compound are sufficiently mixed, to thereby make contact ashomogeneously as possible. In order to attain homogenous contact, boronor a boron compound employed must have as small a particle size aspossible. When the particles are large, a domain containing highconcentration of boron species is locally generated, to thereby possiblyinduce solidification. Specifically, the average particle size is 100 μmor less, preferably 50 μm or less, more preferably 20 μm or less.

[0076] A compound such as boric acid may be added in the form of anaqueous solution, and water may be vaporized in advance, or water may bevaporized during a heating step. If the aqueous solution is mixedhomogeneously, a boron compound can be homogeneously deposited on thesurface of the fibers after removal of water through vaporization.

[0077] As described above, the vapor-grown fine carbon fibers have asmall bulk density, and the as-produced mass has a bulk density ofapproximately 0.01 g/cm³ or less. Even a product obtained throughheat-treating or disintegrate-crush-classify treatment of the mass has abulk density of approximately 0.02-0.08 g/cm³. Such a low bulk densityis,attributed to the fine carbon fibers being easily flocculated. Sincethe vapor-grown fine carbon fibers have high porosity, a considerablylarge-scale furnace is required for heat treatment, to therebydisadvantageously increase facility cost and lower productivity.Accordingly, a method for effectively introducing boron into acarbonaceous material other than a customary one must be developed.

[0078] In order to effectively carry out introduction of boron, contactbetween carbon and boron must be sufficiently maintained. The twocomponents must be homogeneously mixed, to thereby attain sufficientcontact therebetween. Furthermore, there must be prevented separation ofthe two components during heat treatment, which tends to causeunevenness in concentration.

[0079] Although the carbon fibers and boron or a boron compound may bemixed homogeneously and heated, the mixture is preferably treated so asto attain high density and is heated while the density is maintained(solidified) as long as possible. In a preferred mode of the presentinvention, two starting materials are mixed, and the mixture iscompacted with pressure, to thereby solidify the mixture and increasedensity prior to heat treatment.

[0080] No particular limitation is imposed on the method for mixingcarbon fibers and boron or a boron compound, so long as homogeneityis-maintained. Although a variety of commercially available mixers maybe used, a Henschel mixer having a chopper for disintegrating flocks ispreferred as fine carbon fibers are easily flocculated. The startingfibers which are used may be as-produced fibers or fibers treated at1500° C. or lower. Of these, as-produced fibers are preferred, in viewof production cost.

[0081] There may be employed any of a variety of methods for solidifyinga mixture of carbon fibers and boron or a boron compound so as toincrease density and prevent separation of the two components. Forexample, there may be used shaping, granulating, or a method in which amixture is compacted in a crucible into a certain shape. In the case inwhich the shaping method is used, the shaped mixture may assume any ofcolumnar, plate-like, or rectangular parallelepiped form.

[0082] The above mixture which is solidified to have a high density hasa bulk density of 0.05 g/cm³ or more, preferably 0.06 g/cm³ or more.

[0083] When the pressure for compacting the mixture is relieved aftercompletion of shaping, the shaped mixture may be expanded, to therebylower the bulk density. In consideration of such a case, the bulkdensity obtained during compacting is controlled such that thesolidified mixture has a density of 0.05 g/cm³ or more after pressurerelief. When the carbon fibers are placed in a container, the fibers maybe compacted for enhancing the treatment efficiency by use of a pressingplate or the like to thereby attain a bulk density of 0.05 g/cm³ ormore. The fibers may also be heated while it is being compacted.

[0084] The thus-treated fibers; i.e., a mixture of the fibers and boronor a boron compound and having an enhanced bulk density, is then heated.

[0085] In order to introduce boron into a crystal of carbon, the mixturemust be heated at 2000° C. or higher, preferably 2300° C. or higher.When the temperature is less than 2000° C., reactivity between carbonand boron is poor, to thereby prevent introduction of boron. Thetemperature is preferably 2300° C. or higher so as to further accelerateintroduction of boron, enhance the crystallinity of carbon, and,particularly, regulate d₀₀₂ to 0.3385 nm or less for a fiber having adiameter of about 0.15 μm. Although there is no upper limit of thetemperature of heat treatment, the limit is approximately 3200° C. inaccordance with restrictions imposed by an apparatus.

[0086] No particular limitation is imposed on the furnace used in heattreatment so long as the furnace can maintain a target temperature of2000° C. or higher, preferably 2300° C. or higher. Examples of typicallyemployed furnaces include an Acheson furnace, an electric resistancefurnace, and a high-frequency furnace. Alternatively, a powder of amixture or a shaped powder may be heated through direct application ofelectric current.

[0087] The atmosphere for heat treatment is a non-oxidizing atmosphere,preferably an inert gas such as argon. In view of productivity, theheating time is preferably as short as possible. Particularly, heatingfor a long period of time promotes sintering, to thereby lower theyield. Therefore, after the temperature of a core portion of the shapedmixture has reached the target temperature, further maintenance at thetemperature for one hour or less is sufficient.

[0088] Through the heat treatment, the carbon fibers of the presentinvention first attain a d₀₀₂ of 0.3420 nm or less and enhancedcrystallinity. However, Lc (a thickness of the layers in the directionof the C axis of a carbon crystal as determined by the X-ray diffractionmethod) still remains 40 nm or less, more preferably 32 nm or less,which is equal to that of similarly heat-treated B-free carbon fibers.Generally, in graphitizing carbonaceous material, Lc typically increasesas d₀₀₂ decreases. In the carbon fibers having a diameter of about 0.2μm of present invention, Lc does not increase, but remains at 40 nm orless, which is equal to that of similarly heat-treated B-free carbonfibers. In other words, the fibers of the present invention arecharacterized by d₀₀₂ decreasing but Lc not changing drastically.

[0089] Also, as usual vapor growing fibers are heated, the peak at 1580cm⁻¹ of Raman spectrum increases and the peak at 1360 cm⁻¹ decreases,i.e., the R value decreases, finally to about 0.1 to about 0.2 asgraphitizing, but in a boron treated product of the present invention,the R value was 0.5 or more, about 0.7 to about 0.8.

[0090] Moreover, as the peak at 1580 cm⁻¹ became higher, the halfmaximum full width narrowed to 20 to 40 cm⁻¹.

[0091] Along with decrease in d₀₀₂ and increase in the height of peak at1580 cm⁻¹, the electric conductivity increased to 0.01 Ω·cm or less,specifically 0.003 Ω·cm.

[0092] When shaped fibers having a high density obtained through pressmolding or the like are heated, the fibers are partially sintered tothereby form a block. The heat-treated fibers as such cannot serve as afiller added to an electrode or as an electron-emitting material. Thus,the shaped fiber must be disintegrated into a form suitable for afiller.

[0093] In order to do this, when the block is disintegrated, crushed,and classified into a material suitable for a filler, non-fibrous matteris separated simultaneously. Excessive crushing lowers performance of afiller, whereas insufficient crushing results in poor mixing with anelectrode material, to thereby lose the effect of a filler.

[0094] In order to produce a filler having a suitable shape, the blockformed after heat treatment is disintegrated to attain a dimension of 2mm or less, and is further crushed by use of a crusher. Examples of adisintegrating apparatus which may be employed include an ice-crusherand a Rote-plax. Examples of a crusher which may be employed include apulverizer (impact type), a free crusher, and a Microjet. Non-fibrousmatter can be separated through classification, such as gas-flowclassification. The conditions for crushing and classification vary inaccordance with the type of the crusher and operational conditions. Inorder to fully realize a characteristic of a filler, the fiberspreferably have a length of 5-400 μm and an aspect ratio of 10 or more,more preferably 50 or more.

[0095] When the conditions are represented by bulk density aftercrushing and classification, bulk density 0.001 g/cm³-0.2 g/cm³,preferably 0.005 g/cm³-0.15 g/cm³, more preferably 0.01 g/cm³-0.1 g/cm³.When the bulk density is in excess of 0.2 g/cm³, the length of thefibers is as short as 5 μm or less, depending on the fiber diameter, tothereby provide a poor filler effect; whereas when the bulk density isless than 0.001 g/cm³, the length of the fibers is as long as more than400 μm, depending on the fiber diameter, to thereby lower fillingdensity of the filler. The bulk density referred to herein representstapping bulk density obtained from the volume and weight of fiberswhich-are placed in a container and vibrated until the volume reaches analmost constant value.

[0096] The fine carbon fibers of the present invention are added to abattery electrode, to thereby enhance performance of the battery. Thefibers enhance electric conductivity of an electrode plate of batteriessuch as a lithium battery, a lead secondary battery, a polymer battery,and a dry battery, or the performance of a battery requiring anintercalating property. In addition to an effect of enhancingconductivity of these batteries, the fine carbon fibers of the presentinvention, having excellent crystallinity and conductivity, are added toa lithium battery to thereby enhance the charge-discharge capacitythereof, since the fibers exhibit a great intercalating property as acarbonaceous material for a negative electrode. Particularly, a carbonfiber in which d₀₀₂ is 0.3420 nm or less and Lc is 40 nm or lessprovides the above effects to a considerable degree. Even in the case ofboron-containing carbon fibers in which d₀₀₂ and Lc fall outside theabove ranges, crystallinity and conductivity are more excellent than inthe case of boron-free fine carbon fibers. Thus, the fibers are alsoadaptable to the above use.

[0097] The fine carbon fibers are preferably added to an electrode in anamount of 0.1-20 mass %. Amounts in excess of 20 mass % have an effectof decreasing the filling density of carbon in the electrode to therebylower the charge-discharge capacity of the produced battery; whereasamounts less than 0.1 mass % provide a poor effect of addition.

[0098] In order to produce a battery through the addition of the finecarbon fibers, a material for a negative electrode, the fibers, and abinder are sufficiently kneaded to thereby disperse the fibers ashomogeneously as possible. Examples of a material for a negativeelectrode of a lithium battery include graphite powder and meso-phasecarbon microbeads MCMB).

EXAMPLES

[0099] The present invention will next be described in more detail byway of examples, and effects of the carbon fibers of the presentinvention as a filler for an electrode will be elucidated.

[0100] Enhancement of the Crystallization Degree of Fine Carbon Fibers:

Example 1

[0101] Carbon fibers obtained through a known method (e.g., a methoddisclosed in Japanese Patent Application Laid-Open (kokai) No. 7-150419)in which benzene is thermally decomposed in the presence of an organiccompound containing a transition metal was used as a raw material, whichwas subjected to a heat treatment at 1200° C. The resultant flocculatedfibers were disintegrated, to thereby produce carbon fibers having abulk density of 0.02 g/cm³ and a fiber length of 10-100 [μm. Most of thefiber filaments had a fiber diameter of 0.5 μm or less (average fiberdiameter obtained through observation by use of an SEM image is 0.1 to0.2 μm). X-ray diffraction analysis revealed that the fibers had aninterlayer distance between crystal layers represented by d₀₀₂ of 0.3407nm and Lc of 5.6 nm.

[0102] The thus-produced fibers (2.88 kg) and B₄C powder (averageparticle size of 15 μm, 120 g) were sufficiently mixed by use of aHenschel mixer. The mixture was fed in a 50-L pipe-shaped graphitecrucible and pressed by use of a pressurizing plate made of graphite, tothereby adjust the bulk density to 0.075 g/cm³. The mixture wassubjected to a heat treatment at 2900° C. in an Acheson furnace whilethe mixture was pressed by the pressuring plate which simultaneouslyserved as a cap in the crucible. Heating continued for 60 minutes afterthe temperature reached 2900° C.

[0103] After completion of heat treatment, the fibers were cooled,removed from the crucible, roughly disintegrated and then crushed by useof a bantam mill. Non-fibrous matter was separated through gas-flowclassification.

[0104] Although the fiber diameter of the resultant fibers did notchange, the fibers had a length of 5-30 μm and a bulk density of 0.04g/cm³. The boron content and d₀₀₂ and Lc as determined by the X-raydiffraction method are shown in Table 1. The above heat treatment at2900° C. was repeated except that B₄C was not added to carbon fibers.The results of measurement of the above fibers are shown in Table 1 (thedata indicated by Comp. Ex. 1).

Example 2

[0105] Carbon fibers obtained in a similar manner as described inExample 1 was disintegrated and crushed, to thereby produce carbonfibers having a bulk density of 0.05 g/cm³. Most of the fiber filamentshad a fiber length of 10-50 μm and a fiber diameter of average fiberdiameter of 0.06 μm obtained through observation by use of an SEM image.The thus-produced fibers (150 g) and B₄C powder (average particle sizeof 10 μm, 6 g) were sufficiently mixed by use of a Henschel mixer. Themixture was fed in a cylindrical molding apparatus and pressed, tothereby obtain a column formed of carbon fiber having a diameter of 150mm and a bulk density of 0.087 g/cm³.

[0106] The shaped fibers were heated at 2800° C. under an argon flow for60 minutes in a graphite furnace having a heating medium made ofgraphite.

[0107] After completion of heat treatment, the fibers were removed fromthe furnace, simply disintegrated to a dimension of 2 mm or less by useof a mortar, crushed by use of a bantam mill, and subjected to gas-flowclassification. The obtained B-doped fibers had a bulk density of 0.046g/cm³. Most of the fiber filaments had a length of 5-20 μm.

[0108] The boron content and d₀₀₂ and Lc as determined by the X-raydiffraction method are shown in Table 1. The above heat treatment at2800° C. was repeated except that B₄C was not added to the carbonfibers. The results of measurement of the above fibers are shown inTable 1 (the data indicated by Comp. Ex. 2).

Example 3

[0109] Fine carbon fibers used as a raw material were obtained throughthe same known method disclosed in Japanese Patent Application Laid-Open(kokai) No. 7-150419 in which benzene is thermally decomposed in thepresence of an organic compound containing a transition metal. Thecarbon fibers, which were not subjected to a heat treatment, weredisintegrated to a bulk density of 0.01 g/cm³. Most of the fiberfilaments had a fiber diameter of 0.13 μm or less. The thus-producedfibers (200g) and B₄C powder (average particle size of 19 μm, 8 g) weresufficiently mixed by use of a Henschel mixer. The mixture was fed in ashaping machine and pressed to form a cylinder having a diameter of 150mm. The bulk density after the shaping was 0.07 g/cm³.

[0110] The shaped body was placed in a graphite furnace with a graphiteheater and was subjected to a heat treatment at 2800 0C for 60 minutes.

[0111] After the heating, the shaped body was removed, disintegrated toa diameter of 2 mm or less by use of a mortar and then crushed by use ofa bantam mill. Non-fibrous matter was separated through gas-flowclassification. The doped product had a bulk density of 0.03 g/cm³.

[0112] The boron content and d₀₀₂ and Lc as determined by the X-raydiffraction method are shown in Table 1. The above heat treatment at2800° C. was repeated except that B₄C was not added to carbon fibers.The results of measurement of the above fibers are shown in Table 1 (thedata indicated by Comp. Ex. 3). TABLE 1 Boron content d₀₀₂ Lc (mass %)(nm) (nm) Example 1 1.03 0.3380 29.0 Comp. Ex. 1 — 0.3387 31.8 Example 21.10 0.3381 25.0 Comp. Ex. 2 — 0.3398 26.9 Example 3 1.57 0.3382 31.1Comp. Ex. 3 — 0.3401 21.7 Example 4 1.02 0.3395 25.4 Comp. Ex. 4 —0.3405 21.6 Example 5 0.93 0.3376 29.9 Comp. Ex. 5 — 0.3383 30.5

Example 4

[0113] Carbon fibers obtained in the same manner as in Example 1 weredisintegrated and then crushed to have a bulk density of 0.02 g/cm³.Most of fiber filaments had a length of 10-50 μm and an average diameterof about 15 0.04 μm. 3000 g of the fibers was added with 120 g of B₄Chaving an average particle size of 15 μm and the mixture wassufficiently mixed by use of a Henschel mixer. The mixture (88 g) waspacked in a graphite crucible having an inner diameter of 100 mm and aninner 20 length of 150 mm. The bulk density of the packed mixture was0.08 g/cm³.

[0114] The crucible with a cover was placed in a carbon resistancefurnace and heated in an argon flow at 2800° C. for 60 minutes.

[0115] After completion of the heat treatment, the shaped body wasremoved from the furnace and simply dissociated to a dimension of 2 mmor less by use of a mortar. It was then crushed in a bantam mill andclassified by a gas flow and the obtained boron-doped product had a bulkdensity of 0.04 g/cm³. The boron analysis revealed that 1.02 % of boronwas incorporated in the crystals of the fibers. In Table 1, the found ormeasured values of d₀₀₂ and Lc are shown (d₀₀₂=0.3395 nm and Lc=25.4nm). The values of d₀₀₂ and Lc for carbon fiber which was treated in thesame process except that B₄C was not added are shown as ComparativeExample 4 in Table 1 (d₀₀₂ =0.3405 nm and Lc=21.6 nm).

[0116] The powder resistance of the fibers was measured. The measuringmethod was developed by the inventors and is as follows.

[0117] As shown in FIG. 2, a measuring cell comprises a cell 4 having arequtangular of 10 mm×50 mm and a depth of 100 mm, a compressing rod 2and a receiver 3. A certain amount of powder is put in the cell and thecompressing rod 2 is pressed downward to compress the powder.

[0118] While the pressure and volume are measured, a current, 100 mA, ispassed from an electrode 1 set in the direction normal to the pressingdirection and a voltage (E, V) for a 10 mm distance is read by use oftwo measuring terminals 6 extending from the receiver, and theresistance (R, Ωcm) is calculated from the following formula.

R=E/100 (Ωcm)

[0119] The powder resistance depends on the density and the evaluationshould be made between values measured at the same density. In thiscase, the resistances measured at a powder density of 0.8 g/cm³ arecompared.

[0120] The results are shown in Table 2.

[0121] The measured d₀₀₂ of the product was 0.3395 nm and Lc was 5.4 nm.

[0122] For reference, the resistance of a product produced in the samemanner, but B₄C was not added, is shown as Comparative Example 9 inTable 2.

Example 5

[0123] Vapor grown carbon fibers produced in the same manner as inExample 1 but having a diameter of 0.2 μm larger than that of Example 4were heat-treated at 1300° C. and dissociated to a bulk density of 0.05g/cc. The fibers (150 g) and B₄C (6 g) having an average particle sizeof 10 μm were put and mixed in a Hencshel mixer. The powder was shapedto a round column with a size of 150φ×100 mm by use of a cylinder withan inner diameter and a length of 150φ×400 mm and a pressing device. Thebulk density of the body was 0.030 g/cm³, but it was pressed in thecylinder to have a compressed bulk density of 0.08 g/cm³.

[0124] The shaped body was placed in a graphite furnace with a graphiteheater and heated in an argon atmosphere with a temperature elevationrate of 15° C./min. The heating temperature was 2800° C.

[0125] After the heat treatment, the fibers were removed from thefurnace and lightly dissociated to a dimension of 4 mm or less by use ofa mortar. They were crushed in a bantam mill and classified to have abulk density of 0.03 g/cm³. A boron analysis revealed that boron wasincorporated in the crystals of fibers in an amount of 0.93%.

[0126] The powder resistance of the obtained fiber is shown in Table 2.

[0127] The d₀₀₂ of Example 5 is simultaneously shown, which is loweredas that of Example 4. Lc was 29.9 nm. For reference, d₀₀₂ and Lc of aproduct produced in the same manner but B₄C was not added are shown asComparative Example 5 in Table 1. TABLE 2 Powder resistance d₀₀₂ (Ω ·cm) (nm) Com. Ex. 9 0.013 0.3388 (B not added) Fibers of 0.003 0.3395Example 4 Fibers of 0.002 0.3376 Example 5

[0128] Confirmation of Effect of Filler:

Example 6

[0129] In order to study if the powder resistance of a negativeelectrode is really lowered when the fibers of the invention are addedto the electrode, the fibers of Example 4 were added to commerciallyavailable graphite particles (average particle size of 10 μm) and therelationship between the added amount and the powder resistance wasdetermined. Also the relationship between the added amount and thepowder resistance when the fibers of Comparative Example 3, in whichboron was not added, were used, was determined.

[0130] The resistances for the cases when the fibers were not added andwas added in an amount of 3%, 5% and 10% are shown in Table 3. Themethod for measuring the powder resistance was the same as in Example 4,but the bulk density of the fibers was made to 1.5 g/cm³ since they arepreferably compared when the bulk density was set at the same density asthat of electrode.

[0131] Application of the above fibers to an electrode of a lithiumbattery will next be described in an Example.

Example 7

[0132] An electrode was produced from the fibers alone (100%), and theeffect of the fiber of the present invention was first investigated.

[0133] To each of the carbon fibers produced in Examples 1 to 3 andComparative Examples 1 to 3, PVDF (poly(vinylidene fluoride)) was addedin an amount of 3 mass %. Each mixture was pressed onto nickel mesh, tothereby fabricate a working electrode (negative electrode). The batteryperformance was measured by use of Li metal as a counter electrode.LiPF₆ (1 mol) was dissolved in a mixture of ethylene carbonate (EC) anddiethyl carbonate (DEC) (volume ratio 1:1), to thereby prepare anelectrolyte for the battery. The current density during evaluation ofthe battery was 0.2 mA/g.

[0134] The measured discharge capacity of each battery is shown in Table4.

[0135] Electrodes to which the carbon fibers were added will bedescribed in an Example.

Example 8

[0136] Pitch coke was subjected to a heat treatment at 3000° C. tothereby form graphite particles having an average particle size of 16 μm(G1), which was a carbonaceous material serving as a negative electrodematerial of a battery. Graphite particles incorporated with boron duringheat treatment (GB) and B-free graphite particles (G1) were used. GBcontained boron in an amount of 0.98 mass %.

[0137] PVDF (3 mass %) was added to each of GB, G1, and combinations ofGB or G1 to which carbon fibers produced in Example 1 or ComparativeExample 1 were added in an amount of 5 mass %, to thereby prepare aslurry, which was then pressed onto nickel mesh to thereby produce anelectrode plate. The electrolyte, counter electrode, and current densityemployed in this Example are the same as those employed in the previousExample. The measurements of the discharge capacity of each battery areshown in Table 4. In Reference Examples 1 and 2, electrodes wereconstituted exclusively by GB and G1, respectively. The calculateddischarge capacity in Table 4 was obtained through summation of0.95-fold discharge capacity of G1 or GB and 0.05-fold dischargecapacity of carbon fibers. TABLE 3 Fiber addition Powder resistance (Ω ·cm) (%) Fibers with no B Fibers added with B 0 0.06 — 3 0.05 0.01 50.035 0.007 10  0.025 0.004

[0138] TABLE 4 Discharge Discharge capacity capacity DifferenceElectrode (mAh/g) (mAh/g) (found)- material (found) calculated)(calculated) Example 7-(1) Fibers of 311 Ex. 1 Comp. Ex. 7-(1) Fibers of275 Comp. Ex. 1 Example 7-(2) Fibers of 305 Ex. 2 Comp. Ex. 7-(2) Fibersof 272 Comp. Ex. 2 Example 7-(3) Fibers of 308 Ex. 3 Comp. Ex. 7-(3)Fibers of 269 Comp. Ex. 3 Ref. Ex. 1 GB 319 (B-added) Ref. Ex. 2 G1 280Example 8-(1) GB + 332 319 13 fibers of Ex. 1 (5 mass %) Comp. Ex. 8-(1)GB + 321 317  4 fibers of Comp. Ex. 1 (5 mass %) Example 8-(2) G1 + 292282 10 fibers of Ex. 1 (5 mass %) Comp. Ex. 8-(2) G1 + 282 280  2 fibersof Comp. Ex. 1 (5 mass %)

[0139] As is clear from Table 4, when fine carbon fibers were added to acarbonaceous material serving as a negative electrode material, alithium battery having an electrode (negative electrode) formed of theabove electrode material exhibited a discharge capacity higher than thatof the value calculated from the discharge capacity of an electrodematerial and that of carbon fibers. Particularly, boron-incorporatedfine carbon fibers, to which boron was incorporated and treated at hightemperature to enhance crystallinity, exhibited the above effect. Inother words, addition of boron-containing fine carbon fibers to acarbonaceous material serving as a negative electrode materialdefinitely provides a synergistic effect, although the reason thereforhas not been elucidated. The effect is remarkably high whenhigh-crystallinity fine carbon fiber is added.

INDUSTRIAL APPLICABILITY

[0140] The present invention provides fine carbon fibers, having ahigher level of crystallinity which has not conventionally beenrealized, and fine carbon fibers containing boron. High-crystallinityimparts excellent electric conductivity and thermal conductivity tocarbon fibers. Thus, such carbon fibers are excellent material servingas fillers for resin, ceramics, or metal.

[0141] Particularly, the carbon fibers have a high dispersion efficiencydue to small diameter thereof even in a small amount of addition isadded as a filler for an electrode of a battery. Since the fine carbonfibers of the present invention can permit intercalation of significantamounts of lithium ions, the fibers are able to enhance the dischargecapacity of a battery even with a small amount of addition.

1. (Amended) Fine carbon fibers having a fiber diameter of 1 μm or less,an interlayer distance d₀₀₂ between carbon layers as determined by anX-ray diffraction method of 0.335 to 0.342 nm or less and satisfyingd₀₀₂<0.3448-0.0028 (log φ) where φ stands for the diameter of the carbonfibers and the units of d₀₀₂ and 4 are nm, and a thickness Lc of thecrystal in the C axis direction of 40 nm or less.
 2. Fine carbon fibersaccording to claim 1, wherein the interlayer distance d₀₀₂ betweencarbon layers satisfies d₀₀₂<0.3444-0.0028 (log φ).
 3. Fine carbonfibers according to claim 1, wherein the R value of Raman spectrum is0.5 or more and a peak width at maximum peak height of the peak at 1580cm⁻¹ of the spectrum is 20 to 40 cm⁻¹.
 4. Fine carbon fibers having afiber diameter of 1 μm or less and containing boron in crystals in thecarbon fibers.
 5. Fine carbon fibers according to any one of claims 1through 3 containing boron in crystals in the fibers.
 6. Fine carbonfibers according to claim 5, which contain boron in an amount of 0.1-3mass %.
 7. Fine carbon fibers according to claim 5, which have a fiberdiameter of 0.01-1 μm and an aspect ratio of 10 or more.
 8. Fine carbonfibers according to claim 7, which have a powder resistance of 0.01 Ω·cmor less in the direction normal to the pressing direction when thefibers are pressed to a density of 0.8 g/cm³.
 9. Fine carbon fibersaccording to claim 5, which are vapor-grown carbon fibers.
 10. A processfor producing fine carbon fibers which comprises adding fine carbonfibers having a diameter of 1 μm or less with boron or a boron compoundand heat treating the fine carbon fibers to a temperature of 2000° C. orhigher.
 11. A process for producing fine carbon fibers having a diameterof 1 μm or less which comprises adding fine carbon fibers with boron ora boron compound; regulating the bulk density of the fine carbon fibersto 0.05 g/cm³ or more; and heat treating said fine carbon fibers to atemperature of 2000° C. or higher while said bulk density is maintained.12. A process for producing fine carbon fibers according to claim 10 or11, wherein the amount of the boron or boron compound is 0.1-10 mass %in terms of boron atoms with respect to the carbon fibers.
 13. A processfor producing fine carbon fibers according to claim 12, wherein the finecarbon fibers to which the boron is mixed have a diameter of 0.01 to 1μm and an aspect ratio of 10 or more.
 14. A process for producing finecarbon fibers according to claim 13, wherein the fine carbon fibers towhich the boron or boron compound is mixed are a vapor-grown carbonfibers.
 15. A process for producing fine carbon fibers according toclaim 14, wherein the fine carbon fibers prior to said heat treatment towhich the boron or boron compound is to be added are a fired product ofvapor-grown carbon fibers fired after growth.
 16. A process forproducing fine carbon fibers in according to claim 14, wherein the finecarbon fibers prior to said heat treatment to which the boron or boroncompound is to be added are unfired product of vapor-grown carbon fibersnot fired after growth.
 17. A battery electrode containing fine carbonfibers as recited in any one of claims 1 through
 3. 18. A batteryelectrode containing fine carbon fibers as recited in claim
 5. 19. Abattery electrode containing fine carbon fibers as recited in claim 6 or7.
 20. A battery electrode containing fine carbon fibers as recited inclaim 8 or
 9. 21. A battery electrode according to claim 17, wherein thefine carbon fibers are contained in an amount of 0.1 to 20 mass %.
 22. Abattery electrode according to claim 18, wherein the fine carbon fibersare contained in an amount of 0.1 to 20 mass %.
 23. A battery electrodeaccording to claim 19, wherein the fine carbon fibers are contained inan amount of 0.1 to 20 mass %.
 24. A battery electrode according toclaim 20, wherein the fine carbon fibers are contained in an amount of0.1 to 20 mass %.